Change type of CONF_username to the new CONF_TYPE_STR_AMBI.

This is the pathfinding change that proves it's possible for _one_
Conf setting to become Unicode-capable.

That seems like quite a small reward for all the refactoring in the
previous patches this week! But changing over one configuration
setting is enough to get started with: once all the bugs are out of
this one, we can try switching over some more.

Changing the type to CONF_TYPE_STR_AMBI is enough by itself to make
the configuration dialog box write it into Conf as UTF-8, because
conf_editbox_handler automatically checks whether that possibility is
available. However, setting the same Conf entry from the command line
isn't automatic: I had to add code in the handler for the -l
command-line option in cmdline.c.

This commit also doesn't yet handle the _other_ way to specify a
username on the command line: including it as part of the hostname
argument via "putty user@host" or similar. That's more difficult,
because it also requires deciding what to do about UTF-8 in the actual
hostname.

(That looks as if it ought to be possible: Windows should be able to
handle looking up Unicode hostnames if you use GetAddrInfoW() in place
of getaddrinfo(). But plumbing it through everything in between
cmdline.c and windows/network.c is a bigger job than I'm prepared to
do in this proof-of-concept commit.)

unix/plink.c

@@ -823,7 +823,7 @@ int main(int argc, char **argv)
      * If we have no better ideas for the remote username, use the local
      * one, as 'ssh' does.
      */
-    if (conf_get_str(conf, CONF_username)[0] == '\0') {
+    if (conf_get_str_ambi(conf, CONF_username, NULL)[0] == '\0') {
         char *user = get_username();
         if (user) {
             conf_set_str(conf, CONF_username, user);